An automatic transmission in a vehicle usually includes a plurality of planetary gear mechanisms and a plurality of frictional engagement elements such as a clutch and a brake. These frictional engagement elements are selectively engaged in order to switch power transmission paths for transmitting power from a driving source to wheels in a multistage transmission system.
Such a frictional engagement element includes: a plurality of friction plates disposed between two rotor elements (or between a rotor element and a stator element in the case of a brake), the friction plates being configured to alternately engage with the rotor elements; a piston for pressing the friction plates for engagement; and an engaging hydraulic chamber configured to be subject to a hydraulic pressure to move the piston. The hydraulic pressure in the hydraulic chamber is controlled by a hydraulic control mechanism for supplying or discharging hydraulic oil to or from the hydraulic chamber to achieve an appropriate hydraulic pressure. This engages or disengages the frictional engagement element.
In a clutch including a rotatable engaging hydraulic chamber, hydraulic oil remaining in the hydraulic chamber is subject to a centrifugal force, which may move the piston toward the friction plates (in the engagement direction) and cause a dragging resistance on the friction plates out of engagement while the clutch is disengaged. To prevent the dragging resistance on the friction plates, a centrifugally balancing chamber may be disposed opposite to the engaging hydraulic chamber with the piston therebetween. The centrifugally balancing chamber is subject to a hydraulic pressure to cancel the pressure caused by the centrifugal force.
In a clutch including such a centrifugally balancing chamber, it is necessary to keep a high clutch-engaging hydraulic pressure in the engaging hydraulic chamber by blocking an oil flow between the centrifugally balancing chamber and the engaging hydraulic chamber in order to keep the clutch engaged. While the clutch is disengaged, however, it is desirable to connect the two chambers with each other in order to equalize the hydraulic pressure in one chamber with the hydraulic pressure in the other chamber.
For example, as shown in FIG. 9, Patent Literature 1 discloses an automatic transmission including a frictional engagement element 200 having a centrifugally balancing chamber 203 disposed opposite to an engaging hydraulic chamber 202 with a piston 201 therebetween. The centrifugally balancing chamber 203 is connected with the engaging hydraulic chamber 202 via a ball valve 204. In this embodiment, while the clutch is engaged, the ball valve 204 blocks the oil flow from the engaging hydraulic chamber 202 to the centrifugally balancing chamber 203 to keep a high clutch-engaging pressure in the engaging hydraulic chamber 202. When the clutch is disengaged, the ball valve 204 allows the oil flow from the centrifugally balancing chamber 203 to the engaging hydraulic chamber 202 to equalize the hydraulic pressure in the chamber 202 with the hydraulic pressure in the chamber 203.
To improve the comfortability of a vehicle including a multistage automatic transmission at the time of gear change, it is required to shorten a transmission time, that is, a time for engaging the frictional engagement element (a switch time for engagement) or a time for disengaging the frictional engagement element (a switch time for disengagement).
The embodiment of Patent Literature 1 shortens a switch time for engaging a clutch. In the process of engagement of a clutch, the piston starts to move decreasing the volume of the hydraulic oil in the centrifugally balancing chamber, the hydraulic oil in which moves the ball valve. The open ball valve allows the oil flow from the centrifugally balancing chamber to the engaging hydraulic chamber. This facilitates the supply of hydraulic oil to the engaging hydraulic chamber to shorten a switch time for engaging the clutch.
In the process of disengagement of the clutch, however, the ball valve is kept closed for a while after the start of a movement of the piston because of a high hydraulic pressure in the engaging hydraulic chamber and a decreased volume of the engaging hydraulic chamber due to the movement of the piston. The closed ball valve blocks the oil flow from the engaging hydraulic chamber to the centrifugally balancing chamber. The hydraulic oil in the engaging hydraulic chamber is discharged only by a hydraulic control mechanism. This embodiment thus cannot shorten a time for discharging hydraulic oil, that is, a switch time for disengaging the clutch. Since the embodiment cannot shorten a switch time for disengagement (although it can shorten a switch time for engagement), the embodiment cannot shorten a transmission time for gear change involving engaging a clutch while disengaging another.
The delay in the discharge of hydraulic oil from the engaging hydraulic chamber in the process of disengagement using the ball valve occurs not only in a clutch in which the hydraulic oil in the engaging hydraulic chamber is subject to a centrifugal force but also in a brake in which a discharging hydraulic chamber, which is subject to a discharging hydraulic pressure, is disposed opposite to an engaging hydraulic chamber with a piston therebetween.